The present invention relates to an apparatus for mounting a diffusion-limiting membrane for a sensor, and more particularly to an apparatus for mounting a diffusion-limiting membrane for a sensor suitably adapted to stick a diffusion-limiting membrane to be replaced as necessary, to the surface of an enzyme electrode unit of a sensor.
It is known that a physiologic active substance has a characteristic capable of selectively detecting a very complicated organic compound, protein or the like with high sensibility. With attention directed to this characteristic, researches and developments have been made on measurement of such organic compound, protein or the like with the use of an enzyme electrode unit having base electrodes on which a physiologic active substance is immobilized.
In particular, a sensor incorporating the enzyme electrode above-mentioned is suitable for a biochemical analysis of the components of body fluids for clinical analysis or prevention of a disease. This accelerates the researches and developments above-mentioned.
When measuring a target substance with the use of the enzyme electrode unit above-mentioned, the target substance is oxidized or reduced under the presence of a physiologic active substance. The concentration of the target substance is determined by measuring the amount of a substance produced or consumed in such oxidation or reduction. Accordingly, the upper limit of concentration which can be measured, is determined dependent on the amount of a substance provoking such oxidation or reduction, for example the amount of oxygen.
In view of the foregoing, it has been proposed to increase the concentration measuring limit by limiting the penetration rate of a target substance by a diffusion-limiting membrane mounted on the surface of an enzyme-immobilized membrane in/on which a physiologic active substance is immobilized.
By way of example, the following description will describe a measurement of glucose concentration with the use of a glucose sensor.
As shown in FIG. 26, the glucose sensor has a base stand 91, an electrode unit body 92 disposed at a predetermined position of the base stand 91, and a wetting liquid holding portion (not shown) which is press-contacted with the electrode unit body 92 to maintain the same wet and active. The electrode unit body 92 has base electrodes 93, a glucose oxidase immobilized membrane 94 (hereinafter referred to as the GOD immobilized membrane) covering the base electrodes 93, and a diffusion-limiting membrane 95 covering the GOD immobilized membrane 94. The sensor also has a cap 96 for pressing the GOD immobilized membrane 94 and the diffusion-limiting membrane 95 to the surfaces of the base electrodes 93. The cap 96 integrally holds the diffusion-limiting membrane 95.
When a glucose concentration measurement is not to be made, the wetting liquid holding portion may be pressed to the electrode unit body 92 to maintain the electrode unit body 92 wet. This maintains the GOD immobilized membrane 94 active, thus keeping the sensor ready for an accurate measurement of glucose concentration.
When a glucose concentration measurement is to be made, pressing the wetting liquid holding portion to the electrode unit body 92 is released. Then, a target solution to be measured, for example blood, may be dropped on the diffusion-limiting membrane 95. Large-diameter particles such as blood corpuscles are prevented from entering the GOD immobilized membrane 94, and small-diameter particles only reach to the same. Among the small-diameter particles which have reached to the GOD immobilized membrane 94, glucose is oxidized under the presence of glucose oxidase (hereinafter referred to as GOD) and oxygen to produce gluconic acid and hydrogen peroxide. Through the base electrodes 93, an electric signal corresponding to the amount of the hydrogen peroxide thus produced or the remaining oxygen may be taken out. Based on the electric signal thus taken out, an accurate measurement of the concentration of glucose in blood may be made.
However, when continuously making a number of measurements of the concentration of glucose in bloods of a large number of people, it is required to drop each blood on the diffusion-limiting membrane 95 for every measurement. It is also required to replace the diffusion-limiting membrane which has become clogged due to a plurality of measurements, thereby to prevent the glucose penetration ratio from being lowered more than a predetermined rate. Such replacement involves a replacement of the diffusion-limiting membrane 95 integrally mounted on the electrode unit body 92. Further required is a uniform mounting of a new diffusion-limiting membrane 95 on the GOD immobilized membrane 94. This disadvantageously takes a lot of working time. Further, even though the diffusion-limiting membrane is replaced with great care used, the mounting condition may vary more or less to produce variations in measured data.
When the electrode unit body 92 has a small diameter, resulting in decrease in the cap 96, this causes the manual operation for replacement of the diffusion-limiting membrane 95 to be very difficult. This makes the problems above-mentioned more conspicuous.
Moreover, the diffusion-limiting membrane 95 is mounted on the cap 96, requiring a large space for preserving and/or transporting the same.
The description hereinbefore which has discussed mainly the case of measuring the concentration of glucose, may be also applied to the case of measuring the concentrations of other substances in blood, or the case of measuring the concentrations of substances in other body fluids than blood, such as urine.